Like many web apps Xibo uses php sessions to work out whether you are logged in or not. It looks to me like something is getting in the way of that process.
So when you visit a website you get a session id, which your browser sends with every subsequent request. This session_id is used in Xibo to look up your session, whether you are authenticated, etc.
If that session_id is not being sent (or arriving at Xibo for some reason) OR if your useragent is different between requests, you will not get your previous session and will end up back at the login screen.
You would expect a “Different UserAgent” message in your logs if the user agent changed. If your session didn’t match you wouldn’t expect anything in the logs as we’d have no way to work out whether or not that event was worth logging.
As a first step it would be good to verify that the password is definitely what you think it is by following this process: How do I reset the xibo_admin account password?